PHOENIX — Independent Sen. Kyrsten Sinema of Arizona announced Tuesday that she won’t run for a second term after her estrangement from the Democratic Party left her politically homeless and without a clear path to reelection.

Sinema’s announcement comes after Senate Republicans blocked a bipartisan bill to help secure the U.S.-Mexico border and deliver military aid to Ukraine and Israel, which Sinema spent months negotiating. She’d hoped it would be a signature achievement addressing one of Washington’s most intractable challenges as well as a powerful endorsement for her increasingly lonely view that cross-party dealmaking remains possible.

But in the end, Sinema’s border-security ambitions, and her career in Congress, were swallowed by the partisanship that has paralyzed Congress.

“I love Arizona and I am so proud of what we’ve delivered,” she said in a video posted to social media. “Because I choose civility, understanding, listening, working together to get stuff done, I will leave the Senate at the end of this year.”

Sinema’s decision avoids a three-way contest in one of the most closely watched 2024 Senate races, a hard-to-forecast scenario that spawned fierce debate among political operatives about whether one major party would benefit in the quest for the Senate majority. Most analysts agreed Sinema had faced significant, likely insurmountable hurdles if she’d decided to run.

Sinema, the first openly bisexual person elected to the Senate, had raised money for a potential reelection campaign and significantly stepped up her public appearances in Arizona throughout 2023, though her activities slowed as her announcement neared. During her five years in office, she built a formidable campaign bank account pegged at $10.6 million on Dec. 31, 2023, but her quarterly fundraising was outpaced by Democrat Ruben Gallego and Republican Kari Lake.

Sinema was a Democrat for most of her political career but left the party late last year, saying she doesn’t fit into the two-party system. She had alienated many of her colleagues and her party’s base by blocking progressive priorities, often siding with business interests. In an era tribalistic party loyalty, she went out of her way to build relationships with Republicans.

When Sinema became an independent in late 2022, Democrats feared she would split the left-of-center vote and allow a Republican to win.

Republicans have a favorable map in the battle for control of the Senate. Democrats will be forced to defend 23 seats, including Sinema’s and two others held by independents who usually vote with Democrats, compared with just 10 seats for Republicans.

Sinema tried to build her Senate career in the mold of John McCain, the legendary Arizona Republican whose willingness to buck the GOP infuriated his party’s base but endeared him to the state’s more moderate voters.

But she ended up hewing closer to the path of Jeff Flake, a former Arizona Republican senator who stood against then-President Donald Trump and became a pariah in his party. Like Sinema, Flake declined to run for a second term after it became clear he could not survive a primary.

Flake later crossed the aisle to endorse Democrat Joe Biden in 2020 against Trump and was rewarded with an appointment as ambassador to Turkey.

Sinema did not say what the future holds for her. But in her video message announcing her departure, she blamed the current political climate, saying “Americans still choose to retreat farther to their partisan corners.”

“It’s all or nothing,” she said. “The only political victories that matter these days are symbolic, attacking your opponents on cable news or social media.”

Her 2018 election marked the first time in a generation that a Democrat had won a Senate seat from Arizona. It was the start of a period of ascendance for a Democrats in a state long dominated by the GOP.

In Congress, she has been at the center of many of the biggest congressional deals of Biden’s presidency, from a bipartisan infrastructure package to a landmark bill to legally protect same-sex marriages.

She’s also been a reliable vote for Democrats on most nominations and legislation. But with the party hamstrung by razor-thin majorities, she refused to give her blessing to some of the progressive movement’s top priorities.

Her support for maintaining the Senate’s filibuster rule, which requires 60 of 100 votes to pass most legislation instead of a simple majority, has been a particular source of frustration for progressives, who say it gives Republicans a veto despite the Democratic majority. Sinema says it forces the bipartisan compromise that most voters crave.

She single-handedly thwarted her party’s longtime goal of raising taxes on wealthy investors. The year before, she received nearly $1 million from private equity professionals, hedge fund managers and venture capitalists whose taxes would have increased under the plan.

At times, she’s seemed to take delight in serving as a roadblock.

She curtsied while casting a vote against raising the minimum wage. A few weeks later, with backlash to that vote still fresh, she posted to Instagram a photo of herself at brunch wearing a ring that said “(expletive) off.”

Progressives dialed up the pressure. Activists followed her into a bathroom seeking answers to their questions. Critics disrupted a wedding where she was a guest. And the Rev. Jesse Jackson was among demonstrators arrested in a protest outside her Phoenix office.

Long before she faced reelection, donors threatened to walk away, and several groups began collecting money to support an eventual challenger.

In 2022, before she became an independent, leaders of the Arizona Democratic Party formally censured Sinema, a symbolic move that carried no practical impact but was emblematic of the rupture of her relationship with the party.

Sinema’s political career began as a rabble-rousing anti-war activist. A self-described “Prada socialist,” she ran unsuccessfully for local office as a member of the Green Party. She was later elected to the Arizona Legislature as a Democrat and became a prolific spokesperson against Republican bills. Witty, pithy and accessible, she was on speed dial for journalists covering the Legislature.

But she came to believe that she could be more effective building bridges with the Republican majority than publicly excoriating them, she wrote in her 2009 book, “Unite and Conquer.” It was the start of her move toward the center and the persona that has formed her national brand.